1. Field of the Invention
The present invention relates to monoalkyl phosphate-containing polymer dispersions which are used as binders for anticorrosion coatings.
2. Discussion of the Background
Anticorrosion coatings are coating materials whose anticorrosion action is determined on the one hand by the binder and on the other hand by the pigments or fillers.
Today, anticorrosion paints which cause little pollution are being produced to an increasing extent using water-dilutable binders, such as alkyd resin emulsions, solvent-containing alkyd resins which are water-dilutable or water-soluble after neutralization with an amine, and polymer dispersions.
The requirements which have to be met by water-dilutable binders used in anticorrosion paints are that the polymer film swells only slightly in water and that the polymer dispersion in stable to metal ions, eg. zinc ions, so that active anticorrosion pigments, such as zinc phosphate, can be used. Furthermore, the anticorrosion coating should exhibit good adhesion to various metals, especially iron and steel, should be resistant to chalking and weathering and should protect the coated substrate from corrosion.
Since aqueous alkyd resin systems are stable neither to metal ions nor to the effects of weathering and are not resistant to chalking, they are virtually useless for anticorrosion coatings. Moreover, the solvent present in these binder systems in concentrations of up to about 40%, based on solid alkyd resin, has an adverse effect on the environment.
Coating materials based on polymer dispersions dry more rapidly than alkyd resin systems and are more stable to weathering and to chalking, but their stability to metal ions is unsatisfactory in some cases, especially since many polymer dispersions coagulate when active pigments, such as zinc phosphate, are added, because the stability of their emulsifier systems is insufficient. The quality of the corrosion protection afforded by coating materials based on polymer dispersions is determined by the monomer composition of the polymers and the emulsifier system of the polymer dispersion.
Emulsifiers which have proven useful are nonionic compounds, for example oxyethylated alkylphenols and fatty alcohols, and anionic compounds, such as alkylsulfates, sulfated alkyl oxyethylates and mono- and di-phosphates. For example, German Laid-Open Application DOS No. 2,533,043 describes, inter alia, phosphate emulsifiers for the emulsion polymerization of olefinically unsaturated monomers, these emulsifiers being intended to improve the color stability during heating of the films formed from the polymer dispersions. U.S. Pat. No. 4,104,100 discloses polyanionic phosphate emulsifiers for polymer dispersions for anodic electrocoating, these emulsifiers having an advantageous effect on the deposition of the polymer films. Finally, British Pat. No. 1,327,030 describes phosphorus-containing polymer dispersions which are used as binders for strippable coatings having a low pigment content. These strippable coatings must be capable of being removed easily from substrate metals which are temporarily protected by these coatings.